1. Technical Field
The present technology pertains generally to auditory stimuli and detecting electrophysiological signals in response to the stimuli, and more particularly to producing frequency-multiplexed chirp-speech stimuli that can be used with both traditional and advanced techniques to analyze electrical brain activity.
2. Background
Medical diagnosis and treatment of auditory system diseases and deficiencies have been advanced with an increased understanding of the integrated functions of the auditory system components. Generally, sounds presented at the eardrum are transmitted through the three bones of the middle ear to the oval window of the auditory inner ear called the cochlea. The movement of the oval window by the stapes bone pushes and pulls on the enclosed fluid of the cochlea creating compression pressure waves travelling approximately at the speed of sound in water. The motion of the fluid in the cochlear tubes also generates travelling waves along the length of the basilar membrane.
Sensory receptor cells, called hair cells, are positioned on an epithelial ridge on the basilar membrane called the organ of Corti. Motion of the basilar membrane of the cochlea activates the auditory receptor cells (hair cells) sitting on the membrane which send signals through the auditory nerve to the brainstem. The nerve signals activate more neurons and the auditory messages are sent to the midbrain, thalamus, and on to the auditory cortex of the temporal lobe of the cerebrum.
The cochlear travelling wave travels much slower than sound travels in water and peaks at frequency specific locations along the length of the basilar membrane. The basilar membrane is not uniform throughout its length and is relatively narrow but thick at the base and is relatively wide and thin at the apex of the cochlea. As a consequence of the structure, a cochlear travelling wave has a peak amplitude or height of displacement of the basilar membrane at a certain point along its length that is determined by the frequency of the sound that originally produced the motion of the cochlear fluid. High frequencies cause a peak wave near the narrow part of the membrane at the base and low frequencies produce their peaks toward the apex at the broad part of the membrane. The receptor cells on the basilar membrane may also be tuned to particular frequencies, so that each cell responds best to a sound of a given frequency.
The motion of the travelling wave of the basilar membrane is from the base up to the maximum point of displacement and then quickly fades. The latency of the traveling wave is quite short at the base, in the range of tens to hundreds of microseconds, but lengthens dramatically for peak displacements near the apex of the basilar membrane with latencies in the range of 10 ms or greater.
The traveling wave has been considered the fundamental mechanism for analysis of sounds in the cochlea since it was found that the traveling wave amplitude grows linearly with sound intensity and shows a broad peak near the resonant frequency location in the cochlea.
However, a time delay is observed from the initial auditory stimulus at the tympanic membrane and the neural activity produced in the auditory nerve or in the brain stem. It is also observed that the latency to the firing of auditory nerve fibers is comparatively shorter for fibers that are tuned to high frequencies and longer for fibers tuned to lower frequencies.
This latency is caused, in part, by frequency-dependent traveling wave delays. Because a traveling wave takes a period of time to reach from the base to the apical end of the cochlea, the corresponding receptor cells and fibers of the auditory nerve are not stimulated simultaneously. Consequently, synchronization between the nerve fibers is decreased due to the delays introduced by the traveling wave. The temporal dispersion or asynchrony of the collective output from the different neural elements results in a reduction of the amplitude of the corresponding compound neural response (e.g. ACAP or ABR).
The temporal asynchrony between the neural elements can be partially compensated for with a short auditory stimulus such as an upward chirp, where the higher frequencies are delayed relative to the lower frequencies. By aligning the arrival time of each frequency component of the stimulus, the chirp stimulus can shorten the latency and shift the waves. Essentially, the structure of the stimulus can influence the deflections of the basilar membrane and the corresponding timing of the stimulation of the hair cells of the basilar membrane and auditory nerve of the inner ear.
Auditory stimuli such as clicks, chirps or tone pulses have become part of several different procedures in the auditory field such as the auditory brain stem response (ABR), auditory compound action potential (ACAP), and the auditory steady-state response (ASSR). For example, hearing screening in newborns based on auditory brainstem response (ABR) is a well established method for the early detection of hearing impairment.
Several models have been developed that attempt to capture the major features of cochlear behavior. Nevertheless, there are many aspects of the human auditory system that are not fully understood. For example, it is not understood how different features of a sound, such as frequency, intensity, or onset and offset that are carried to higher brain centers separately through parallel nerve pathways are interpreted by the cerebral cortex.
Speech is perhaps the most important stimulus that humans encounter in their daily lives. Unfortunately, certain populations have difficulty understanding speech, particularly in noisy conditions, including children with language-based learning problems and users of cochlear-implant/hearing-aids. Even in the hearing impaired, much of this difficulty stems not from problems in the ear itself, but from how each individual's brain processes the speech sounds.
Considering its importance, audiology clinics devote little effort to the neural processing of continuous speech from the ear to auditory cortex. Currently there is no way to rapidly and reliably assess the integrated functioning of the auditory system from brainstem to cortex. For this profound, global health problem, no clinical assessment tool exists. Accordingly, there is a need for devices and methods that will allow the diagnosis, treatment and study of human hearing.